Jaundice due to unconjugated hyperbilirubinemia in the newborn can lead to serious physiologic changes including cerebral palsy and death. The simplest, now most prevalent but not completely understood for this jaundiced condition is phototherapy. During phototherapy unconjugated bilirubin (UCB) interacts with light and is both excreted as UCB and degraded to water-soluble products. The jaundiced condition is thus alleviated until the infant can begin to excrete its bilirubin. We have begun a program to investigate the interaction of bilirubin and light and have generated considerable new information. We propose to examine in detail the mechanism of UCB excretion during phototherapy and to determine the structures and possible toxicity of the photodegradation products. We will also investigate bilirubin photochemistry in the presence and absence of oxygen and under a variety of solvent or phase conditions which mimic the in vivo situation. We are especially interested, inter alia, in distinguishing the Z yields reversibly E isomerization (main) pathway from photooxidation pathways, isolating the (presumed) photooxidation products from urine, and determining the importance of new photoproducts (X, Y and Z).